"Fuel cell," for purposes of this specification, is understood to be an electrochemical cell in which the free energy of combustion of the fuel cell is converted directly into electrical energy. Fuel cells have been recognized as a very efficient means for the direct conversion of the chemical energy of a fuel, including coal, hydrocarbons and products of their processing, into electricity. Not being thermal engines, they are not limited by the Carnot Cycle, and their efficiency is in principle high. In addition, since they are modular this efficiency, as well as the cost per unit of power, is broadly independent of size. Furthermore, the efficiency is not substantially degraded when operated at levels which are substantially lower than the rated power, and then any degradation is not due to the fuel cell proper. Fuel cells tend to be silent, with noise resultant from ancillary equipment only. Generally, fuel cells are non-polluting.
Because of all the aforesaid advantages, fuel cells have been the subject of considerable developmental work. For the electric utilities, fuel cells have characteristics which enable them to compete in a broad spectrum of applications from disperse, peak power plants, on the order of 10-20 megawatts, now served by inefficient, short life gas turbines to baseload applications of 1000 megawatts and more which are now served by very capital intensive coal and nuclear plants.
For the gas utilities, fuel cells are ideal for highly dispersed power plants with co-generation capabilities, operating on natural gas, or equivalent, for apartment buildings and industrial complexes. The size for the envisioned applications presently vary from 40 kilowatts to 200 kilowatts.
Another application for fuel cells, which is at an early state of development, is for electric automobile traction, with methanol, or possibly ethanol, as the primary fuel which may be reformed prior to use in the fuel cell. As a result of these applications there is a large interest in fuel cells.
A fuel cell is essentially a simple device and comprises as the only basic components a housing, fuel and oxidant electrodes, and an electrolyte which can be immobilized within a porous matrix. The fuel and oxygen electrodes of the cell are generally constructed as porous, planar members with one surface maintained in contact with the electrolyte while the fuel and oxygen gases are caused to come in contact with the other surface of the electrode. As the fuel and oxygen are passed through or in contact with the electrodes, they are reacted at the surface of the respective electrode.
In constructing an efficient fuel cell, the problem encountered is basically one of electrochemical or chemical kinetics, the object being to carry out the reaction of the fuel and oxidizing gas in such a manner that the proportion of free energy degraded into heat is as small as possible. Yet, it is necessary that the activity of the cell be sufficiently high so that the energy output from practical sized cells is economically high.
A large amount of the research has been expended upon the catalytic surface of the electrodes and to a lesser extent in developing more efficient electrolytes. The early work in the electrolyte area has been directed toward the use of solid and fused electrolytes such as the alkali and alkaline earth carbonates. Further, in the development of low and medium temperature fuel cells, aqueous electrolytes of alkali hydroxides including eutectic mixtures of the hydroxides with some water have received considerable attention. In the process of dissociation, these strongly basic materials produce large numbers of hydroxyl ions which readily transfer oxygen ions to the fuel electrode where, as for example, hydrogen ions react with oxygen or hydroxyl ions to form molecules of water.
For an efficient fuel cell, it is necessary that the electrolyte remain invariant and have a high ionic conductivity. When the electrolyte undergoes chemical change through reaction with the fuel, with impurities such as CO.sub.2 in the air, or oxidation by the oxidizing gas, it is necessary to replenish or exchange the electrolyte in order to maintain the high activity and corresponding high current density of the fuel cell.
Most of the fuel cell development for commercial applications has been based on the phosphoric acid electrolyte fuel cell system, often described as the "first generation" commercial fuel cell system, in which the fuel utilized is the effluent gases from a hydrocarbon reformer or coal gasifier, which contain hydrogen, carbon dioxide and carbon monoxide, are passed through the anode of a phosphoric acid fuel cell at which the hydrogen is electrochemically oxidized. The cell operates at temperatures of 170.degree. C.-210.degree. C. and pressures of up to 10 Atm, depending on the application. Two complete 4.8 megawatt systems, fabricated by United Technologies Corporation, have been installed in New York City and Tokyo to demonstrate the application of the fuel cell for peak power production in electric utility applications. Of these two installations, the Tokyo one has been successfully operated.
In spite of the engineering advances made on the phosphoric acid fuel cell systems, a number of problems in the fuel cell stack have been recognized. The most important are (a) limitations in materials of construction, e.g., bipolar plates, which are expensive and prone to corrosion, especially at high cell voltages; (b) the relatively low voltage efficiency of the cell, which limits the total system efficiency, or "heat rate," an important consideration for electric utilities especially for the baseload application, and (c) the limitation in electrocatalyst selection to only platinum. Even at the high degree of dispersion presently obtainable of about 0.5 g/1000 cm.sup.2, this limitation still represents a hindrance to the widespread introduction of fuel cells.
For all these reasons, even before the "first generation" phosphoric acid fuel cell has been commercially deployed, development on a "second generation fuel cell"--the molten carbonate fuel cell--has been underway. The materials demand for the molten carbonate fuel cell are quite staggering since it requires inexpensive components capable of operating at about 650.degree. C. under strong oxidizing and/or reducing conditions. In particular, no cathode yet exists capable of showing thousands of hours of operation under pressure. Also, the cell anode is highly sensitive to hydrogen sulfide poisoning.
Although attention has been directed to aqueous alkaline or alkali cells, a particularly serious problem is encountered when carbonaceous fuels are used since carbon dioxide is produced as a by-product in the fuel processing or in the direct electrochemical oxidation of the fuel. This practically precludes the use of electrolytes containing alkali metal hydroxides, or other compositions which form insoluble carbonates. The formation of insoluble carbonates within the cell raises the melting point of the electrolyte and reduces the overall efficiency of the cell by increasing the internal resistance and/or by blocking the porous electrodes with insoluble matter, interrupting the continuity of the liquid electrolyte layer between the two electrodes and eventually causing a catastrophic failure. It is apparent from thermodynamic considerations that fuel cells utilizing carbonaceous fuels do not necessarily require high operating temperatures. The use of high temperatures in the prior art cells generally result from the nature of the solid or molten electrolyte used.
There is a need, therefore, for a fuel cell capable of rejecting carbon dioxide, which does not suffer from the serious materials problems shown by the phosphoric acid and the higher temperature cells, and which operates at an equal or higher cell efficiency than the phosphoric acid cell, and at no added capital cost. It is also desirable that the new cell be able to utilize the efforts invested in the development of other systems, particularly the phosphoric acid electrolyte technology, including the development of non-fuel cell components such as fuel processor and power conditioner, and permit use of the electrodes developed therefor, such as the Teflon-bonded gas-diffusion electrode structures. It is also desirable that any new fuel cell or fuel cell stack be able to retrofit presently designed plants, such as the phosphoric acid electrolyte plants.